Finalize head inheritance

Finalize head inheritance

* hydra.el (hydra--body-exit): New defun.
(defhydra): Ensure that each head doesn't need the :exit info from the
body any more by putting the aggregated :exit in the head's own plist.

* hydra-test.el: Update tests.

Each hydra will now declare its own heads as a variable `foo/heads`.
It's possible to inherit them like this:

(defhydra hydra-zoom-child (:inherit (hydra-zoom/heads))
  "zoom"
  ("q" nil))

One hydra can inherit from multiple parents. This one just adds a single
"q" head to the familiar hydra-zoom.

Fixes #57.